Seat belt devices are known which effectively restrain and protect a vehicle occupant by retracting a webbing through a pretensioner in a case where a side-slippage of a vehicle body, slipping of vehicle wheels, or the like occurs at the time of driving (for example, see Patent Document 1). In such seat belt devices, a control unit determines the state of a side-slippage of a vehicle based on detection values acquired by a wheel-speed sensor, a yaw rate sensor, a lateral acceleration sensor, a steering angle sensor, and the like.
In addition, a seat belt device that uses a motor as a driving source of the pretensioner is known.